In communication systems, a signal is modulated and amplified for transmission over communication channels. The signal is modulated using a modulator and amplified using a linear power amplifier. Different modulation techniques are developed to convey the signal across the communication channel. Examples of modulation techniques include amplitude modulation, phase modulation, and frequency modulation. Depending upon the modulation technique used, a resultant transmitted signal may have a varying dynamic range and a varying Peak to Average Power Ratio (PAPR). The dynamic range is defined as the ratio between the peak signal power and the minimum signal power. The PAPR is defined as the ratio between peak signal power and the average signal power. The dynamic range and the PAPR of the modulated signal affect the efficiency of the linear power amplifier. Efficiency of the linear power amplifier decreases as the dynamic range, the PAPR, or a combination of both, increases.
In other words, amplitude value of the peak signal and the minimum signal affects the efficiency of the linear power amplifier. Higher amplitude of the peak signal, and lower amplitude of the minimum signal reduce the efficiency of the linear power amplifier. Therefore, in order to increase the efficiency of the linear power amplifier, either the amplitude of the minimum signal may be enlarged, or the amplitude of the peak signal may be suppressed.
There are various methods that use suppression or enlargement of the amplitude of the peak signal or minimum signal to improve the efficiency of the linear power amplifier. A window clipping scheme is one such method that uses suppression of the amplitude of the peak signal of a modulated signal to increase the efficiency of the linear power amplifier. On the other hand, a windowed minimum enlarger increases the amplitude of the minimum signal. The windowed minimum enlarger is applied to signals having minimum amplitude values that are less than a predefined minimum threshold value. The windowed minimum enlarger function scales-up the amplitude of that portion of the signal.
Adjacent Channel Coupled Signal Power (ACCPR) is defined as the ratio of the signal power in the adjacent channel and the signal power in the main channel. In order to maintain a low ACCPR for a multi-carrier signal, both the window clipping and the windowed minimum enlarger have to be applied with a considerable window length. The windowed minimum enlarger is the same with the window clipping but clipping the low peak (or minimum) signal. For example, to maintain a low ACCPR a window size of 19 samples is used in a High Performance Data (HPD) signal. This causes overlap in the samples of window clipping and the windowed minimum enlarger. Further, the amplification factor for minimum enlargement is more than that used for peak suppression. As a result of this overlap, the suppressed peak signals lying in the overlapping region are again increased by the windowed minimum enlarger, which increases the Peak to Average Power Ratio (PAPR) again.
Accordingly, there is a need for an improved apparatus and technique for conditioning a linear modulation signal without interfering with peak suppression schemes.
Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of embodiments of the present invention.